1. Technical Field
The present invention relates to a balanced-unbalanced conversion circuit having a filtering function of attenuating interference waves, in addition to a balanced-unbalanced conversion function.
2. Related Art
Balanced-unbalanced conversion circuits (Balun) have been used for connection between an unbalanced type device and a balanced type device and between devices having different impedance. Recently, a circuit provided with a trap circuit has been proposed to prevent interference waves from being transmitted to a rear end circuit in various electronic apparatuses such as communication apparatuses.
FIG. 9 is a schematic circuit diagram illustrating the known balun. As shown in FIG. 9, the balun 100 mainly includes an unbalanced terminal 101, a first coupling line 102 connected between the unbalanced terminal 101 and a reference potential (ground), two balanced terminals 103 and 104, and a second coupling line 105 connected between the balanced terminals 103 and 104 and coupled to the first coupling line 102. A trap circuit includes an LC parallel resonant circuit 106 connected between one end of the second coupling line 105 and one balanced terminal 103, and an LC parallel resonant circuit 107 connected between the other end of the second coupling line 105 and the other balanced terminal 104. The LC parallel resonant circuits 106 and 107 establish attenuation poles in a frequency band adjacent to a pass band.
According to the balun 100 with such a configuration, an unbalanced signal input from the unbalanced terminal 101 is converted into a balanced signal between the first coupling line 102 and the second coupling line 105, and the converted signal is output from the balanced terminals 103 and 104. In this case, interference waves are removed by the trap circuit (106, 107) having the attenuation poles in a frequency band of the interference waves, so as to prevent the interference waves from being transmitted to a rear end circuit 108.
As shown with a dotted line in FIG. 9, the interference waves included in the signal transmitted from the second coupling line 105 to the balanced terminals 103 and 104 are removed by the trap circuit (106, 107), but the interference waves from the ground terminal of the first coupling line 102 cannot be prevented from being input to the rear end circuit 108 through the ground. Particularly, at high frequency, it is difficult to make a ground having invariable potential, and thus it is important to provide for the prevention of the interference waves transmitted through the ground.